For seismic surveys on land, geophones are usually hand "planted" or placed individually on the ground and are connected by clip leads or plugs, to a long multiconductor seismic cable. The number of geophones used per survey station may total eight hundred to two thousand such instruments. Geophones are constructed to be responsive to vertically-travelling compressional seismic waves, but are generally insensitive to seismic waves travelling at right angles to the vertical axis of the geophone. Typically a geophone has one direction of maximum sensitivity. That direction is designed to be vertical. The sensitivity falls off at increasing angles from the direction of maximum sensitivity. If the axis of the geophone is tilted substantially away from the vertical, the output signals from the geophone are attenuated or distorted.
In shallow-water environments such as lakes, swamps, rivers, and bays, it is not practical to plant geophones on the bottom beneath the water. Accordingly, hydrophones are substituted for geophones. Since hydrophones respond to pressure waves rather than to vertically-travelling compressional waves, hydrophones have no preferred orientation. Instead of hand-planting the orientation-insensitive hydrophones, they are permanently secured to the seismic cable. The entire cable, including hydrophones, is then dragged from station to station. Twisting of the cable and random hydrophone orientation are of no consequence.
In the conduct of a line of survey that includes alternately both dry and submerged landforms, geophones are customarily used on dry land. Over water covered areas, a hydrophone "drag cable" is substitued for the land cable and geophones. This causes problems, as it is most undesirable to substitute a pressure-sensitive detector for a motion-sensitive detector. Corresponding signals from each of the two types of detectors are out of phase with each other and are difficult, if not impossible, to correlate, one with the other. Because of this fact, it is preferred to use the same type of detector for the entire survey. Furthermore, the sensitivity of hydrophones in very shallow water is inferior to that of a geophone under the same conditions.
Employing marine survey techniques, it would be useful to permanently secure geophones to the seismic cable. Not only would this practice eliminate the need for hand-planting a thousand or more geophones at each station, it would also be possible to use the same detector for both land and shallow marine operations. The problem of a geophone being sensitive principally to seismic waves received along its axis has, up to the present time, prevented the widespread use of geophones integrally mounted in seismic cables.
Two principal alternatives have been proposed heretofore to solve the foregoing problem. In accordance with one of these alternatives, geophones have been secured to a relatively flat belt, as shown in U.S. Pat. No. 3,825,866 for example, and the orientation of the flat belt is maintained as it is dragged along the ground or in shallow water by the flat geometry of the belt. Another well-known alternative involves the use of gimbal-mounted geophones, in which vertical orientation of the geophones is obtained by mounting them pendulously for free orientation under the force of gravity.
Neither of the foregoing two alternatives is as satisfactory as would be desirable, as the flat belts are unduly heavy, bulky, and awkward; and the mechanical complexity of the gimbal mountings as well as the fact that the geophone is coupled to the ground through the gimbal mountings with consequent signal distortion, makes them expensive and subject to mechanical failure in the field and inferior in the quality of seismic signal output.
Accordingly, an important object of the present invention is to provide an improved cable-mounted geophone arrangement.